THE HUMAN MACHINE 



269 



is an example of a good habit, round shoulders a bad habit of this 

 sort. The habit of a wrong position of bones and muscles once 

 formed is very hard to correct. 

 This can best be done by certain 

 corrective exercises at home or 

 in the gymnasium. 



Round shoulders is most com- 

 mon among people whose occu- 

 pation causes them to stoop. 

 Drawing, writing, and a wrong 

 position when at one's desk are 

 among the causes. Exercises 

 which strengthen the back 

 muscles and cause the head to 

 be kept erect are helpful in form- 

 ing the habit of erect carriage. 



Slight curvature of the spine 

 either backward or forward is 



helped most by exercises which Diagram showing action of biceps muscle, 

 tend to Straighten the body, a. contracted; b, extended; h, humerus; 



? -XT. xlT S ' SCa P ula ' 



such as stretching up with the 



hands above the head. Lateral curvature of the spine, too often 



caused by a " hunched-up " position at the school desk, may also 



A %r B C 



Three classes of levers in the human body; bones and muscles act together. 

 A, a lever of the first class; B, a lever of the second class ; C, a lever of the 

 third class. 



be corrected by exercises which tend to lengthen the spinal 

 column. 

 It is the duty of every girl and boy to have good posture and 



